What happens every 823 years isn't merely an occurrence of five Fridays, Saturdays and Sundays in a month. It's the occurrence of five very particular Fridays, Saturdays and Sundays. The ones which will occur 823 years from now, in 2833, will be exactly the same as the ones which will occur this month. It's like the movie Groundhog Day but on a grander and more convoluted timescale and, thankfully, without Bill Murray.

People (assuming there are still people around by then) will awaken on Friday, October 1, 2833, and discover they've been transported backward in time to 2010. They will discover that their abilities to communicate telepathically have disappeared, and they will have to learn how to blog and send text messages. All five Fridays, Saturdays and Sundays during the month will be affected by this chronospatial anomaly, though the four Mondays, Tuesdays, Wednesdays and Thursdays will not. This will make for some remarkably confused folks by November first. It will take a month's rest, some really good drugs, and a sizable stimulus package from the Galactic Overlord before they return to normal.

Then it'll all happen again in 3656.